The present invention relates generally to a method and system of geophysical exploration and more particularly to a method and system for obtaining a composition log of the minerals comprising the earth's subterranean formations surrounding a wellbore.
In the continuing search for oil and gas, explorationists have developed a wide array of geophysical exploration methods for imaging the earth's subterranean formation structures and displaying those images as well as providing estimates of selected formation characteristics such as lithology, porosity, density, and velocity of propagation of seismic energy in the earth's formations. Exemplary of such geophysical exploration methods are seismic surveys, gravity surveys, magnetic surveys, acoustic logging, resistivity logging and neutron logging.
Occasionally, core samples are obtained from regions of interest within a limited portion of the wellbore for transport to a central laboratory so that properties of the formations adjacent the borehole, such as fluid content of the sample, (U.S. Pat. No. 2,458,093), oil content of the sample (U.S. Pat. No. 2,531,083), and resistivity of the sample (U.S. Pat. No. 2,613,250) can be obtained. Additionally, analyses of such core samples to determine mineralogy have been undertaken at remote laboratory facilities. Unfortunately, such core samples generally represent only a small fraction of the formation lithologies traversed by the wellbore along its length and as such the measured properties represent only a small fraction of the earth's formations surrounding the wellbore.
During the drilling process, chips which have broken away from the subsurface formations can be captured from the drilling fluid at the surface for analysis. Typically, explorationists have identified the lithologies of the subsurface formations from such chips. However, determining the exact depth in the wellbore from which the chips originated is generally not possible due to the very nature of their transport to the surface. Consequently, core samples and chip samples fail to provide either a regular sampling of borehole formation lithologies along the length of the wellbore or fail to accurately determine the depth of origin of such samples so that logs of formation characteristics, as a function of depth over generally the entire wellbore length, cannot be produced. Moreover, the very nature of the chip samples can often times preclude measuring certain formation characteristics (for example, measurement of seismic velocity or compressive strength).
In view of such difficulties, the present invention provides a novel method of geophysical exploration whereby a composition log of the minerals comprising the formations surrounding a wellbore can be obtained. With such composition logs, explorationists can better evaluate the results of geophysical surveys and thus better evaluate the oil- and gas-bearing potential of such formations.